Receptors, Messenger Molecules, Agonists, and Antagonists
The surfaces of nerve cells in the central nervous system (the CNS, which includes the brain, spinal cord, and retina) contain various types of receptor molecules. In general, a receptor molecule is a polypeptide which straddles a cell membrane. When a messenger molecule interacts with the exposed extracellular portion of the membrane receptor molecule, it triggers a difference in the electrochemical status of the intracellular portion of the receptor, which in turn provokes some response by the cell. The messenger molecule does not bond to the receptor; instead, it usually disengages from the receptor after a brief period and returns to the extracellular fluid. Most receptor molecules are named according to the messenger molecules which bind to them.
An "agonist" is any molecule, including the naturally occurring messenger molecule, which can temporarily bind to and activate a certain type of receptor. An agonist can cause the same effect as the natural messenger molecule, or in some cases it can cause a more intense effect (for example, if it has a tighter affinity for the receptor molecule and remains bound to the receptor for a prolonged period).
By contrast, an "antagonist" is a molecule which can block or reduce the effects exerted by the natural messenger molecule. This can happen in several different ways. A "competitive antagonist" binds to a certain type of receptor without triggering it, thereby preventing the natural messenger molecule from reaching and activating the receptor. A "non-competitive antagonist" functions in other ways. For example, a receptor referred to as the PCP receptor, which is triggered by molecules such as PCP or MK-801, apparently can override the effects of a different type of receptor, the NMDA receptor (both receptors are discussed below). Therefore, PCP and MK-801 are regarded as non-competitive antagonists for the NMDA receptor.
The role a certain molecule plays as an agonist or antagonist must be viewed with regard to a certain type of receptor. For example, while MK-801 is an antagonist for the NMDA receptor, it is an agonist for the PCP receptor. Most agonists and antagonists are xenobiotic drugs, i.e., they do not exist naturally in the body.
For more information on neuroanatomy, neurotransmitters, receptors molecules, and agonists and antagonists which interact with CNS receptors, see Adelman 1987 (complete citations are provided below).
The two main classes of excitatory receptor molecules are referred to as "cholinergic" receptors and "glutamate" receptors. Both types of receptors are present in the synaptic junctions that serve as pathways for impulses between CNS nerve cells. Most other types of receptors in the CNS involve inhibitory neurotransmitters.